1. Field of the Invention
This invention relates to a catalyst for automobiles that is used for purifying an exhaust gas emitted from an internal combustion engine of an automobile.
2. Description of the Related Art
To protect the environment, the requirements for purification of exhaust gases have been increasing in recent years. To cope with the problem, a combustion control for an internal combustion engine to control emission of pollutants and a catalytic process has been developed. A three way catalyst has gained a wide application in the past as a catalyst for such a catalyst system. The three way catalyst efficiently purifies HC, CO and NOx in the operating window of a stoichiometric air to fuel ratio. As the air to fuel ratio must be controlled within a relatively narrow range in the three way catalyst, systems that use a NOx catalyst having improved NOx reduction performance, or combine a plurality of catalysts, have been tested in a lean-burn engine and a direct injection type gasoline engine.
As shown in FIG. 8 of the accompanying drawings, these catalytic bodies generally use a monolithic catalyst support 101 formed of a highly heat-resisting ceramic such as cordierite, and include a coating layer 103 of a material having a large specific area such as γ-alumina, on its cell walls 102, to support catalytic components thereon. The main catalytic component is generally a catalyst precious metal 104 such as Pt or Rh, and an assistant catalyst 105 is further supported depending on the application. In the case of the NOx catalyst, for example, the catalyst further includes a NOx absorption material as the assistant catalyst 105 so that NOx absorbed in a lean atmosphere is emitted in a rich atmosphere, and is reduced and purified by HC and CO on the catalyst precious metal.
To reduce exhaust emission, it is desired to activate the catalyst system quickly and to prevent emission of the contaminants at the start of the engine. To achieve quick activation, it is effective to reduce the thermal capacity of the catalyst. For example, a proposal has been made that decreases the wall thickness of the catalyst support to reduce the thermal capacity of the catalyst and arranges this catalyst as a start catalyst at an upstream portion of the catalyst system to thus shorten the time required for the temperature rise of the catalyst.
In the catalysts according to the prior art, however, the coating layer 103 must be formed to support the catalytic component. Therefore, reduction of the thermal capacity is limited when merely decreasing the thickness of the cell walls 102 of the catalyst support 101. As the coating layer 103 is formed, the open area of the cells decreases with the result that a pressure loss increases.
On the other hand, a catalytic body capable of supporting a catalyst without forming a coating layer 103 has been examined. For example, a catalytic body that increases a specific surface area of a ceramic support by the application of acid treatment is known. Since this ceramic body still involves the problem of the strength, European Patent Application EP1043067A2 proposes a ceramic catalytic body that creates crystal defects or fine pores consisting of fine cracks on a ceramic surface to impart a catalyst supporting capability to a ceramic support, and supports catalytic components on the ceramic support.